For the VOC it is:

VOC = E_g - something,

where something is positive and depends on the Urbach energy. Otherwise it should be linear: https://aip.scitation.org/doi/pdf/10.1063/1.49397

For the ISC I didn't find anything. Only one paper on bifacial solar cells, where there Equation (4) gives a formula, but I am not sure if this is helpful for the "general" case (I am writing about CIGS solar cells right now, and a general Theory part, not specifically about CIGS): https://www.researchgate.net/publication/327345077_Thermodynamic_Efficiency_Limit_of_Bifacial_Solar_Cells_for_Various_Spectral_Albedos/figures?lo=1&utm_source=google&utm_medium=organic

Can someone elaborate, please?